Why Evolution Is True (11 page)

Second, when we find transitional forms, they occur in the fossil record precisely where they should. The earliest birds appear after dinosaurs but before modern birds. We see ancestral whales spanning the gap between their own landlubber ancestors and fully modern whales. If evolution were not true, fossils would not occur in an order that makes evolutionary sense. Asked what observation could conceivably
disprove
evolution, the curmudgeonly biologist J. B. S. Haldane reportedly growled, “Fossil rabbits in the Precambrian!” (That’s the geological period that ended 543 million years ago.) Needless to say, no Precambrian rabbits, or any other anachronistic fossils, have ever been found.

Finally, evolutionary change, even of a major sort, nearly always involves remodeling the old into the new. The legs of land animals are variations on the stout limbs of ancestral fish. The tiny middle ear bones of mammals are remodeled jawbones of their reptilian ancestors. The wings of birds were fashioned from the legs of dinosaurs. And whales are stretched-out land animals whose forelimbs have become paddles and whose nostrils have moved atop their head.

There is no reason why a celestial designer, fashioning organisms from scratch like an architect designs buildings, should make new species by remodeling the features of existing ones. Each species could be constructed from the ground up. But natural selection can act only by changing what already exists. It can’t produce new traits out of thin air. Darwinism predicts, then, that new species will be modified versions of older ones. The fossil record amply confirms this prediction.

Chapter 3

Remnants: Vestiges, Embryos, and Bad Design

—Theodosius Dobzhansky

 

 

 

I
n medieval Europe, before there was paper, manuscripts were made by writing on parchment and vellum, thin sheets of dried animal skin. Because these were hard to produce, many medieval writers simply reused earlier texts by scraping off the old words and writing on the newly cleaned pages. These recycled manuscripts are called
palimpsests,
from the Greek
palimpsestos,
meaning “scraped again.”

Often, however, minute traces of the earlier writing remained. This has proved critical in our understanding of the ancient world. Many ancient texts are in fact known to us only by peering beneath the stratum of medieval overwriting to recover the original words. Perhaps the most famous of these is the Archimedes Palimpsest, first written in Constantinople in the tenth century and then cleaned and overwritten three centuries later by a monk making a prayer book. In 1906, a Danish classicist identified the original text as the work of Archimedes. Since then, a combination of X-rays, optical character recognition, and other complex methods have been used to decipher the original underlying text. This painstaking work yielded three mathematical treatises of Archimedes written in ancient Greek, two of them previously unknown and enormously important in the history of science. In such arcane ways we recover the past.

Like these ancient texts, organisms are palimpsests of history-evolutionary history. Within the bodies of animals and plants lie clues to their ancestry, clues that are testimony to evolution. And they are many. Hidden here are special features, “vestigial organs,” that make sense only as remnants of traits that were once useful in an ancestor. Sometimes we find “atavisms”—throwback traits produced by the occasional reawakening of ancestral genes that have long been silenced. Now that we can read DNA sequences directly, we find that species are also
molecular
palimpsests: in their genomes is inscribed much of their evolutionary history, including the wrecks of genes that once were useful. What’s more, in their development from embryos, many species go through contortions of form that are bizarre: organs and other features appear, and then change dramatically or even disappear completely before birth. And species aren’t all that well designed, either: many of them show imperfections that are signs not of celestial engineering but of evolution.

Stephen Jay Gould called these biological palimpsests the “senseless signs of history.” But they are not really senseless, for they constitute some of the most powerful evidence for evolution.

As A GRADUATE STUDENT IN BOSTON, I was enlisted to help a senior scientist who had written a paper about whether it was more efficient for warm-blooded animals to run on two legs or four. He planned to submit the paper to Nature, one of the most prestigious scientific journals, and asked me to help him take a photograph striking enough to land on the journal cover and call attention to his work. Eager to get out of the laboratory, I spent an entire afternoon chasing a horse and an ostrich around a corral, hoping to get them to run side by side, demonstrating both types of running in a single frame. Needless to say, the animals refused to cooperate, and, all species being exhausted, we finally gave up. Although we never got the picture,
¹⁴
the experience did teach me a biology lesson: ostriches can’t fly, but they can still use their wings. When they’re running, they use their wings for balance, extending them to the sides to keep from toppling over. And when an ostrich becomes agitated—as it tends to do when you chase it around a corral—it runs straight at you, extending its wings in a threat display. That’s a sign to get out of the way, for a miffed ostrich can easily disembowel you with one swift kick. They also use their wings in mating displays,
¹⁵
and spread them out to shade their chicks from the harsh African sun.

The lesson, though, goes deeper. The wings of the ostrich are a
vestigial trait:
a feature of a species that was an adaptation in its ancestors, but that has either lost its usefulness completely or, as in the ostrich, has been co-opted for new uses. Like all flightless birds, ostriches are descended from flying ancestors. We know this from both fossil evidence and from the pattern of ancestry that flightless birds carry in their DNA. But the wings, though still present, can no longer help the birds take flight to forage or escape predators and bothersome graduate students. Yet the wings are not useless—they’ve evolved new functions. They help the bird maintain balance, mate, and threaten its enemies.

The African ostrich isn’t the only flightless bird. Besides the
ratites—
the large flightless birds that include the South American rhea, the Australian emu, and the New Zealand kiwi-dozens of other bird species have independently lost the ability to fly. These include flightless rails, grebes, ducks, and, of course, penguins. Perhaps the most bizarre is the New Zealand kakapo, a tubby flightless parrot that lives mainly on the ground but can also climb trees and “parachute” gently to the forest floor. Kakapos are critically endangered: fewer than one hundred still exist in the wild. Because they can’t fly, they are easy prey for introduced predators like cats and rats.

All flightless birds have wings. In some, like the kiwi, the wings are so small-only a few inches long and buried beneath their feathers—that they don’t seem to have any function. They’re just remnants. In others, as we saw with the ostrich, the wings have new uses. In penguins, the ancestral wings have evolved into flippers, allowing the bird to swim underwater with amazing speed. Yet they all have exactly the same bones that we see in the wings of species that can fly. That’s because the wings of flightless birds weren’t the product of deliberate design (why would a creator use exactly the same bones in flying and flightless wings, including the wings of swimming penguins ?), but of evolution from flying ancestors.

Opponents of evolution always raise the same argument when vestigial traits are cited as evidence for evolution. “The features are
not
useless,” they say. “They are either useful for something, or we haven’t yet discovered what they’re for.” They claim, in other words, that a trait can’t be vestigial if it still has a function, or a function yet to be found.

But this rejoinder misses the point. Evolutionary theory doesn’t say that vestigial characteristics have no function. A trait can be vestigial and functional at the same time. It is vestigial not because it’s functionless, but because
it no longer performs the function for which
it evolved
. The wings of an ostrich are useful, but that doesn’t mean that they tell us nothing about evolution. Wouldn’t it be odd if a creator helped an ostrich balance itself by giving it appendages that just happen to look exactly like reduced wings, and which are constructed in exactly the same way as wings used for flying?

Indeed, we expect that ancestral features will evolve new uses: that’s just what happens when evolution builds new traits from old ones. Darwin himself noted that “an organ rendered, during changed habits of life, useless or injurious for one purpose, might easily be modified and used for another purpose.”

But even when we’ve established that a trait is vestigial, the questions don’t end. In which ancestors was it functional? What was it used for? Why did it lose function? Why is it still there instead of having disappeared completely ? And which new functions, if any, has it evolved?

Let’s take wings again. Obviously, there are many advantages to having wings, advantages shared by the flying ancestors of flightless birds. So why did some species lose their ability to fly? We’re not absolutely sure, but we do have some powerful clues. Most of the birds that evolved flightlessness did so on islands—the extinct dodo on Mauritius, the Hawaiian rail, the kakapo and kiwi in New Zealand, and the many flightless birds named after the islands they inhabit (the Samoan wood rail, the Gough Island moorhen, the Auckland Island teal, and so on). As we’ll see in the next chapter, one of the notable features of remote islands is their lack of mammals and reptiles—species that prey on birds. But what about ratites that live on continents, like ostriches? All of these evolved in the Southern Hemisphere, where there were far fewer mammalian predators than in the north.

The long and short of it is this: flight is metabolically expensive, using up a lot of energy that could otherwise be diverted to reproduction. If you’re flying mainly to stay away from predators, but predators are often missing on islands, or if food is readily obtained on the ground, as it can be on islands (which often lack many trees), then why do you need fully functioning wings? In such a situation, birds with reduced wings would have a reproductive advantage, and natural selection could favor flightlessness. Also, wings are large appendages that are easily injured. If they’re unnecessary, you can avoid injury by reducing them. In both situations, selection would directly favor mutations that led to progressively smaller wings, resulting in an inability to fly.

So why haven’t they disappeared completely? In some cases they nearly have: the wings of the kiwi are functionless nubs. But when the wings have assumed new uses, as in the ostrich, they will be maintained by natural selection, though in a form that doesn’t allow flight. In other species, wings may be in the process of disappearing, and we’re simply seeing them in the middle of this process.

Vestigial eyes are also common. Many animals, including burrowers and cave dwellers, live in complete darkness, but we know from constructing evolutionary trees that they descended from species that lived aboveground and had functioning eyes. Like wings, eyes are a burden when you don’t need them. They take energy to build, and can be easily injured. So any mutations that favored their loss would clearly be advantageous when it’s just too dark to see. Alternatively, mutations that reduced vision could simply accumulate over time if they neither helped nor hurt the animal.

Just such an evolutionary loss of eyes occurred in the ancestor of the eastern Mediterranean blind mole rat. This is a long, cylindrical rodent with stubby legs, resembling a fur-covered salami with a tiny mouth. This creature spends its entire life underground. Yet it still retains a vestige of an eye—a tiny organ only one millimeter across and completely hidden beneath a protective layer of skin. The remnant eye can’t form images. Molecular evidence tells us that, around 25 million years ago, blind mole rats evolved from sighted rodents, and their withered eyes attest to this ancestry. But why do these remnants remain at all? Recent studies show that they contain a photopigment that is sensitive to low levels of light, and helps regulate the animal’s daily rhythm of activity. This residual function, driven by small amounts of light that penetrate underground, could explain the persistence of vestigial eyes.

True moles, which are not rodents but insectivores, have independently lost their eyes, retaining only a vestigial, skin-covered organ that you can see by pushing aside the fur on its head. Similarly, in some burrowing snakes the eyes are completely hidden beneath the scales. Many cave animals also have eyes that are reduced or missing. These include fish (like the blind cave fish you can buy at pet stores), spiders, salamanders, shrimp, and beetles. There is even a blind cave crayfish that still has eyestallcs, but no eyes atop them!

Whales are treasure troves of vestigial organs. Many living species have a vestigial pelvis and leg bones, testifying, as we saw in the last chapter, to their descent from four-legged terrestrial ancestors. If you look at a complete whale skeleton in a museum, you’ll often see the tiny hindlimb and pelvic bones hanging from the rest of the skeleton, suspended by wires. That’s because in living whales they’re not connected to the rest of the bones, but are simply imbedded in tissue. They once were part of the skeleton, but became disconnected and tiny when they were no longer needed. The list of vestigial organs in animals could fill a large catalog. Darwin himself, an avid beetle collector in his youth, pointed out that some flightless beetles still have vestiges of wings beneath their fused wing covers (the beetle’s “shell”).